Optical isolation module

ABSTRACT

An optical source for a photolithography tool includes a source configured to emit a first beam of light and a second beam of light, the first beam of light having a first wavelength, and the second beam of light having a second wavelength, the first and second wavelengths being different; an amplifier configured to amplify the first beam of light and the second beam of light to produce, respectively, a first amplified light beam and a second amplified light beam; and an optical isolator between the source and the amplifier, the optical isolator including: a plurality of dichroic optical elements, and an optical modulator between two of the dichroic optical elements.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No. 14/970,402, filed Dec. 15, 2015, now allowed, and titled OPTICAL ISOLATION MODULE, which claims the benefit of U.S. Provisional Application No. 62/236,056, filed Oct. 1, 2015, and titled OPTICAL ISOLATION MODULE. Both of these prior applications are incorporated herein by reference in their entirety.

TECHNICAL FIELD

This disclosure relates to an optical isolation module. The optical isolation module can be used in an extreme ultraviolet (EUV) light source.

BACKGROUND

Extreme ultraviolet (“EUV”) light, for example, electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13 nm, can be used in photolithography processes to produce extremely small features in substrates, for example, silicon wafers.

Methods to produce EUV light include, but are not necessarily limited to, converting a material that has an element, for example, xenon, lithium, or tin, with an emission line in the EUV range in a plasma state. In one such method, often termed laser produced plasma (“LPP”), the required plasma can be produced by irradiating a target material, for example, in the form of a droplet, plate, tape, stream, or cluster of material, with an amplified light beam that can be referred to as a drive laser. For this process, the plasma is typically produced in a sealed vessel, for example, a vacuum chamber, and monitored using various types of metrology equipment.

SUMMARY

In one general aspect, an optical source for a photolithography tool includes a source configured to emit a first beam of light and a second beam of light, the first beam of light having a first wavelength, and the second beam of light having a second wavelength, the first and second wavelengths being different; an amplifier configured to amplify the first beam of light and the second beam of light to produce, respectively, a first amplified light beam and a second amplified light beam; and an optical isolator between the source and the amplifier, the optical isolator including: a plurality of dichroic optical elements, and an optical modulator between two of the dichroic optical elements.

Implementations can include one or more of the following features. The optical modulator can include an acousto-optic modulator. Each of the dichroic optical elements can be configured to reflect light having the first wavelength and to transmit light having the second wavelength; and the acousto-optic modulator can be positioned on a beam path between two of the dichroic optical elements, the acousto-optic modulator can be positioned to receive reflected light from the two of the dichroic optical elements, the acousto-optic modulator can be configured to transmit the received light when the received light propagates in a first direction relative to the acousto-optic modulator and to deflect the received light away from the beam path when the received light propagates in a second direction relative to the acousto-optic modulator, the second direction being different from the first direction. The first and second beams of light can be pulsed beams of light. An energy of the first amplified light beam can be less than an energy of the second amplified light beam. The first amplified light beam can have an energy sufficient to deform target material in a target material droplet into a modified target, the modified target including target material in a geometric distribution that is different than a distribution of the target material in the target material droplet, the target material including material that emits extreme ultraviolet (EUV) light when in a plasma state, and the second amplified light beam has an energy sufficient to convert at least some of the target material in the modified target to the plasma that emits EUV light.

The acousto-optic modulator can be positioned on a beam path between two of the dichroic optical elements and can be positioned to receive light reflected from the two of the dichroic optical elements, the acousto-optic modulator can be configured to receive a trigger signal, and the acousto-optic modulator can be configured to deflect received light from the beam path in response to receiving the trigger signal, and to otherwise transmit received light onto the beam path.

The optical source also can include a second optical modulator between the source and the amplifier. The second optical modulator is between two of the dichroic optical elements, and the second optical modulator is on a different beam path than the optical modulator.

The source can include a laser source. The source can include a plurality of sources, the first light beam being produced by one of the sources, and the second light beam being produced by another one of the sources. The source can include one or more pre-amplifiers.

In another general aspect, an apparatus for an extreme ultraviolet (EUV) light source includes a plurality of dichroic optical elements, each of the dichroic optical elements being configured to reflect light having a wavelength in a first band of wavelengths and to transmit light having a wavelength in a second band of wavelengths; and an optical modulator positioned on a beam path between two of the dichroic optical elements, the optical modulator positioned to receive reflected light from the two dichroic optical elements, and the optical modulator configured to transmit the received light when the received light propagates in a first direction on the beam path and to deflect the received light away from the beam path when the received light propagates in a second direction on the beam path, the second direction being different from the first direction, where the first band of wavelengths includes a wavelength of a pre-pulse beam, and the second band of wavelengths includes a wavelength of a main beam.

Implementations can include one or more of the following features. The optical modulator can be an acousto-optic modulator. The apparatus also can include a control system configured to provide a trigger signal to the acousto-optic modulator, and the acousto-optic modulator can be configured to deflect light away from the beam path in response to receiving the trigger signal and otherwise transmits light onto the beam path.

The apparatus also can include a second optical modulator, where the second optical modulator is between two of the dichroic optical elements, and the second optical modulator is positioned to receive light transmitted by the two dichroic optical elements. The optical modulator and the second optical modulator can be between the same two dichroic optical elements, and the second optical modulator can be on a second beam path that is different from the beam path.

In another general aspect, a method includes reflecting a first beam of light at a first dichroic optical element, the reflected first beam of light passing through an optical modulator and an amplifier to produce an amplified first light beam; transmitting a second beam of light through the first dichroic optical element, a second dichroic optical element, and the amplifier to produce an amplified second beam; receiving a reflection of the amplified first light beam at the second dichroic optical element, wherein an interaction between the reflection of the amplified first light beam and the second dichroic optical element directing the reflected amplified first light beam to the optical modulator; and deflecting the reflection of the amplified first light beam at the optical modulator to thereby direct the reflection of the amplified first light beam away from a source of the first beam of light.

Implementations can include one or more of the following features. A trigger signal can be provided to the optical modulator after the first beam of light passes through the optical modulator and before the reflection of the amplified first light beam is at the optical modulator. The trigger signal can cause the optical modulator to be in a state in which the optical modulator deflects incident light.

The amplified first light beam can propagate toward an initial target region. The reflection of the first amplified light beam can be produced through an interaction between the first amplified light beam and a target material droplet in the initial target region. The second amplified light beam can propagate toward a target region, and an interaction between target material and the second amplified light beam can produce a reflection of the second amplified light beam, the method further including: transmitting the reflection of the second amplified light beam through the second dichroic optical element, and deflecting the reflection of the second amplified light beam at a second optical modulator to thereby direct the reflection of the second amplified light beam away from a source of the second beam of light. The source of the first beam of light and the source of the second beam of light can be the same source. The source of the first beam of light can be a first optical subsystem in the source, and the source of the second beam of light can be a second optical subsystem in the source.

Implementations of any of the techniques described above may include a method, a process, an optical isolator, a kit or pre-assembled system for retrofitting an existing EUV light source, or an apparatus. The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are block diagrams of exemplary optical systems.

FIGS. 3 and 6 are block diagrams of exemplary optical isolators.

FIGS. 4A and 4B are block diagrams of exemplary optical arrangements that can be used in the optical isolators of FIGS. 3 and 6.

FIGS. 5A and 5B are timing plots associated with an exemplary optical modulator.

FIG. 7 is a block diagram of an exemplary control system.

FIGS. 8A and 8B are a block diagram of a drive laser system for an extreme ultraviolet (EUV) light source.

FIGS. 9, 10A, 10B, 11A, 11B, 12A-12C, and 13A-13C are examples of experimental data collected with and without an optical isolator.

DETAILED DESCRIPTION

Referring to FIG. 1, a block diagram of an exemplary optical system 100 is shown. The optical system 100 is part of an extreme ultraviolet (EUV) light source. The optical system 100 includes an optical source 102 that produces a light beam 110. The light beam 110 is emitted from the optical source 102 and propagates along a path 112 in a direction z toward a target region 115.

The target region 115 receives a target 120, which includes material that emits EUV light when converted to plasma. The target 120 is reflective at the wavelength or wavelengths of the light beam 110. Because the target 120 is reflective, when the light beam 110 interacts with the target 120, all or part of the beam 110 can be reflected along the path 112 in a direction that is different from the z direction. The reflected portion of the beam 110 is labeled as the reflection 113. The reflection 113 can travel on the path 112 in a direction that is opposite to the z direction and back into the optical source 102. Reflections of a forward-going beam (a beam that propagates from the optical source 102 toward the target region 115), such as the reflection 113, are referred to as “back reflections.”

The optical source 102 includes a light-generation module 104, an optical isolator 106, and an optical amplifier 108. The light-generation module 104 is a source of light (such as one or more lasers, lamps, or any combination of such elements). The optical amplifier 108 has a gain medium (not shown), which is on the beam path 112. When the gain medium is excited, the gain medium provides photons to the light beam 110, amplifying the light beam 110 to produce the amplified light beam 110. The optical amplifier 108 can include more than one optical amplifier arranged with the respective gain mediums on the path 112. The optical amplifier 108 can be all or part of a drive laser system, such as the drive laser system 880 of FIG. 8B.

The light-generation module 104 emits the light beam 110 onto the beam path 112 toward the optical isolator 106. The optical isolator 106 passes the light beam 110 in the z direction to the optical amplifier 108 and toward the target region 115. However, the optical isolator 106 blocks the back reflection 113. Thus, and as discussed in greater detail below, the optical isolator 106 prevents the back reflection from entering the light-generation module 104. By preventing the back reflection from entering the light-generation module 104, additional optical power can be delivered to the target 120, which can lead to an increase in the amount of generated EUV light.

Referring to FIG. 2, a block diagram of an EUV light source 200 that includes an exemplary optical source 202 is shown. The optical source 202 can be used in place of the optical source 102 in the optical system 100 (FIG. 1). The optical source 202 includes a light-generation module 204, which includes two optical subsystems 204 a, 204 b, the optical amplifier 108, and the optical isolator 106. The optical isolator 106 is on the path 112 and between the optical amplifier 108 and the light-generation module 204.

The optical subsystems 204 a, 204 b produce first and second light beams 210 a, 210 b, respectively. In the example of FIG. 2, the first light beam 210 a is represented by a solid line and the second light beam 210 b is represented by a dashed line. The optical subsystems 204 a, 204 b can be, for example, two lasers. In the example of FIG. 2, the optical subsystems 204 a, 204 b are two carbon dioxide (CO₂) lasers. However, in other implementations, the optical subsystems 204 a, 204 b are different types of lasers. For example, the optical subsystem 204 a can be a solid state laser, and the optical subsystem 204 b can be a CO₂ laser.

The first and second light beams 210 a, 210 b have different wavelengths. For example, in implementations in which the optical subsystems 204 a, 204 b include two CO₂ lasers, the wavelength of the first light beam 210 a can be about 10.26 micrometers (μm) and the wavelength of the second light beam 210 b can be between 10.18 μm and 10.26 μm. The wavelength of the second light beam 210 b can be about 10.59 μm. In these implementations, the light beams 210 a, 210 b are generated from different lines of the CO₂ laser, resulting in the light beams 210 a, 210 b having different wavelengths even though both beams are generated from the same type of source. The light beams 210 a, 210 b also can have different energies.

The light-generation module 204 also includes a beam combiner 209, which directs the first and second beams 210 a, 210 b onto the beam path 112. The beam combiner 209 can be any optical element or a collection of optical elements capable of directing the first and second beams 210 a, 210 b onto the beam path 112. For example, the beam combiner 209 can be a collection of mirrors, some of which are positioned to direct the first beam 210 a onto the beam path 112 and others of which are positioned to direct the second beam 210 b onto the beam path 112. The light-generation module 204 also can include a pre-amplifier 207, which amplifies the first and second beams 210 a, 210 b within the light-generation module 204.

The first and second beams 210 a, 210 b can propagate on the path 112 at different times, but the first and second beams 210 a, 210 b follow the path 112 and both beams 210 a, 210 b traverse substantially the same spatial region to the optical isolator 106, and through the optical amplifier 108. As discussed with respect to FIGS. 3 and 6, the first and second beams 210 a, 210 b are separated within the optical isolator 106, and then propagate on the path 112 to the optical amplifier 108.

The first and second beams 210 a, 210 b are angularly disbursed by a beam delivery system 225 such that the first beam 210 a is directed toward an initial target region 215 a, and the second beam 210 b is directed toward a modified target region 215 b, which is displaced in the −y direction relative to the initial target region 215 a. In some implementations, the beam delivery system 225 also focuses the first and second beams 210 a, 210 b to locations within or near the initial and modified target regions 215 a, 215 b, respectively.

In the example shown in FIG. 2, the initial target region 215 a receives an initial target 220 a and the first beam 210 a. The first beam 210 a has an energy that is sufficient to modify the geometric distribution of target material in the initial target 220 a (or to initiate the spatial reconfiguration of the target material) into a modified target that is received in the modified target region 215 b. The second beam 210 b is also received in the modified target region 215 b. The second beam 210 b has an energy that is sufficient to convert at least some of the target material in the modified target 220 b into a plasma that emits EUV light. In this example, the first beam 210 a can be referred to as a “pre-pulse”, and the second beam 210 b can be referred to as the “main pulse.”

The first beam 210 a can reflect off of the initial target 220 a, giving rise to a back reflection 213 a that can propagate along the path 112 in a direction other than the z direction and into the optical amplifier 108. Because the first beam 210 a is used to modify a spatial characteristic of the initial target 220 a and is not intended to convert the initial target 220 a into the plasma that emits EUV light, the first beam 210 a has a lower energy than the second beam 210 b. However, reflections of the first light beam 210 a can have more energy than reflections of the second light beam 201 b.

The first beam 210 a (and the reflection 213 a) propagates through the optical amplifier 108 before the second beam 210 b. Thus, the gain medium of the optical amplifier 108 can still be excited when the reflection 213 a passes through the gain medium of the optical amplifier 108. As a result, the reflection 213 a can be amplified by the amplifier 108. Further, the initial target 220 a can be substantially spherical in shape, dense, and highly reflective, whereas the modified target 220 b can be a disk-like shape (or other non-spherical shape), less dense and less reflective. Due to the non-spherical shape, the modified target 220 b can be positioned to reduce the amount of light that reflects back onto the path 112 due to an interaction between the second beam 210 b and the modified target 220 b. For example, the modified target 220 b can be tilted in the x-z and/or y-z plane relative to the direction of propagation of the light beam 210 b, or the modified target 220 b can be away from the focus of the second beam 210 b.

In some implementations, the modified target 220 b is not tilted in the x-z and/or y-z plane, and the modified target 220 b is instead oriented such that the side of the modified target 220 b that has the greatest spatial extent is in a plane that is perpendicular to the direction of propagation of the second beam 210 b. Orienting the modified target 220 b in this manner (which can be referred to as a “flat” target orientation) can enhance the absorption of the second beam 210 b. In some implementations, such an orientation can increase the absorption of the second beam 210 b by about 10% as compared to instances in which the modified target 220 b is tilted 20 degrees (°) relative to a plane that is perpendicular to the direction of propagation of the second beam 210 b. Orienting the modified target 220 b in a flat orientation can increase the amount of reflected light that propagates back into the optical source 202. However, because the optical source 202 includes the optical isolator 106, the modified target 220 b can have a flat orientation because the optical isolator 106 acts to reduce the impact of reflections that can arise from the modified target 220 b in a flat orientation.

Finally, because the second beam 210 b has a relatively large energy, the forward propagation of the second beam 210 b through the amplifier 108 saturates the gain medium, leaving little energy that the amplifier 108 can provide to a back reflection of the second beam 210 b. As such, even though the first beam 210 a has a lower energy than the second beam 210 b, the back reflection 213 a, which arises from the first beam 210 a, can be substantial and can be larger than a back reflection arising from the second beam 210 b.

As discussed below, the optical isolator 106 prevents back reflections arising from the first beam 210 a from entering the light-generation module 204. The optical isolator 106 also can prevent back reflections arising from the second beam 210 b from entering the light-generation module 204, and an example of such an implementation is shown in FIG. 6. Because the optical isolator 106 prevents potentially damaging back reflections from reaching the light-generation module 204, higher energy light beams can be generated from the light-generation module 204, resulting in more energy being delivered to the modified target 220 b and more EUV light. In some implementations, the average amount of EUV light produced can be increased by about 20% by using the optical isolator 106.

Referring to FIG. 3, a block diagram of an exemplary optical isolator 306 is shown. The optical isolator 306 can be used as the optical isolator 106 in the optical source 102 (FIG. 1), the optical source 202 (FIG. 2), or in any other optical source. The optical isolator 306 is discussed with respect to the optical source 202.

The optical isolator 306 includes a dichroic optical element 331, reflective elements 332, an optical modulator 335, and a dichroic element 336. The optical isolator 306 also can include optical arrangements 333, 334. The dichroic elements 331 and 336 are on the beam path 112. The dichroic elements 331 and 336 can be any optical component that is capable of separating or filtering light according to its wavelength. For example, the dichroic elements 331 and 336 can be dichroic mirrors, dichroic filters, dichroic beam splitters, or a combination of such elements. The dichroic elements 331 and 336 can be identical to each other, or they can have different configurations. In the example of FIG. 3, the dichroic elements 331 and 336 reflect the wavelength (or wavelengths) of the first beam 210 a and transmit the wavelength (or wavelengths) of the second beam 210 b.

The first beam 210 a is reflected from the dichroic element 331 onto a beam path 314, which is between the dichroic elements 331 and 336 and has a spatial extent and form defined by the reflective elements 332. The beam path 314 is different from the beam path 112. Thus, in the optical isolator 306, the first beam 210 a does not remain on the beam path 112, and the first and second beams 210 a, 210 b are spatially separated from each other. The first beam 210 a propagates on the beam path 314 through the optical arrangements 333, 334, and the optical modulator 335, before reaching the dichroic element 336, which reflects the beam 210 a back onto the beam path 112. The second beam 210 b passes through the dichroic element 331 and through the dichroic element 336, remaining on the beam path 112 while propagating through the optical isolator 306.

The optical modulator 335 is on the beam path 314 between the dichroic elements 331 and 336. The optical modulator 335 is an optical element that is capable of deflecting incident light away from the path 314. The optical modulator 335 is adjustable between an open state and a closed state such that the optical modulator 335 can transmit the first beam 210 a and block the reflection 213 a (the reflection of the first beam 210 a from the initial target 220 a).

The optical modulator 335 can be, for example, an acousto-optic modulator (AOM). An acousto-optic modulator includes a medium (such as quartz or glass) connected to a transducer (such as a piezo-electric transducer). Motion of the transducer causes sound waves to form in the medium, creating a spatially varying index of refraction in the medium. When the medium includes the sound waves, light incident on the medium is deflected. When the sound waves are not present in the medium, the acousto-optic modulator transmits incident light without deflection. Other optical modulators can be used as the modulator 335. For example, the optical modulator 335 can be a Faraday rotator or an electro-optic modulator (EOM). The modulator 335 can be a combination of such devices, and can include more than one of the same type of device.

In implementations in which the optical modulator 335 is an acousto-optic modulator, the transducer moves at a time when the reflection 213 a is expected to enter the path 314. At other times, the transducer is not moved or vibrated. Thus, the beam 210 a (the forward-going “pre-pulse”) passes through the optical modulator 335, remaining on the path 314 and ultimately rejoining the path 112. However, the reflection 213 a is deflected (shown as deflection 217 a in FIG. 3) away from the path 314. As a result, the reflection 213 a does not reach the light-generation module 204 (FIG. 2).

Because the optical modulator 335 can be configured to transmit incident light only at certain times, the optical isolator 306 provides a time-gate based isolation technique as opposed to one that is based on polarization. Additionally, the optical isolator 306 can be used in combination with a polarization-based isolation technique. For example, the polarization of the back reflections can be different than the polarization of the forward-going beams 210 a, 210 b, and a polarization isolator 303, which includes a polarizing element (such as a thin film polarizer), can be placed between the optical isolator 306 and the optical amplifier 108 (FIGS. 1 and 2) to provide additional blocking of back reflections. The polarizing element of the polarization isolator 303 can be configured to primarily reject reflections of the second light beam 210 b, allowing the optical isolator 306 to be tailored to reject reflections of the first light beam 210 a. By using different techniques to reject reflections of the first light beam 210 a and the second light beam 210 b, the overall amount of reflections reaching the light-generation module 204 from any source can be reduced.

In some implementations, the optical isolator 306 includes first and second optical arrangements 333, 334. The first beam 210 a passes through the first optical arrangement 333 before reaching the optical modulator 335. The first optical arrangement 333 can be any optical element or a collection of optical elements that reduces the beam diameter of the first light beam 210 a. After passing through the optical modulator 335, the first beam 210 a passes through the second optical arrangement 334. The second optical arrangement 334 can be any optical element or a collection of optical elements that enlarge the beam diameter of the second light beam 210 b. The speed at which the optical modulator 335 can be transitioned between being opened (in a state in which incident light is transmitted by the optical modulator 335) or closed (in a state in which incident light is deflected or blocked by the optical modulator 335) increases as the beam diameter decreases. Thus, by reducing the diameter of the first beam 210 a, the first optical arrangement 333 allows the optical modulator 335 to switch between being opened and closed, and vice versa, more quickly than in implementations that lack the first optical arrangement 333. In some implementations, the beam diameter of the beam 210 a can be reduced to about 3 millimeters (mm).

The second optical arrangement 334 enlarges the diameter of the first light beam 210 a prior to directing the first light beam 210 a onto the path 112. Additionally, the second optical arrangement 334 reduces the beam diameter of the reflection 213 a before the reflection 213 a reaches the optical modulator 335. By reducing the beam diameter of the reflection 213 a, the speed at which the optical modulator 335 must be transitioned between the open and closed states to block the reflection 213 a is reduced.

Referring to FIGS. 4A and 4B, block diagrams of exemplary optical arrangements 433 and 434, respectively, are shown. The optical arrangements 433, 434 can be used as the optical arrangement 333, 334, respectively, in the optical isolator 306 (FIG. 3). The optical arrangements 433, 434 are Galilean telescopes, which have one convex lens and one concave lens. In the optical arrangement 433, a concave lens 442 is between a convex lens 441 and the optical modulator 335. In the optical arrangement 434, a concave lens 443 is between the optical modulator 335 and a convex lens 444. Both of the arrangements 433, 434 reduce the diameter of a beam that propagates toward the optical modulator 335. When the optical arrangements 433, 434 are used together in the configuration shown in FIG. 3, the beam diameter of the beam 210 a is reduced prior to being incident on the optical modulator 335, and the beam diameter of the beam 210 a is enlarged by the optical arrangement 434 after passing through the optical modulator 335. The beam diameter of the reflection 213 a is reduced by the optical arrangement 434 prior to reaching the optical modulator 335. The reflection 213 a does not pass through the optical arrangement 433 because the optical modulator 335 deflects the reflection 213 a from the beam path 314.

The optical arrangements 433 and 434 can be identical Galilean telescopes or the arrangements 433 and 434 can includes lenses that have different characteristics (such as different focal lengths).

Referring to FIG. 5A, an exemplary plot that shows the state of the optical modulator 335 as a function of time is shown. FIG. 5B shows the relative placement of pulses of a beam 510 a and a reflection 513 a on the same time axis that is shown in FIG. 5A. The pulse 510 a is a pulse of a beam that propagates through the system 200 (FIG. 2) when the system 200 is configured to use the optical isolator 306 (FIG. 3) as the optical isolator 106, and the reflection 513 a is a reflection of the pulse 513 a from the initial target 220 a. The pulse 510 a is a pulse of a pulsed light beam that is used as a “pre-pulse” to shape the initial target 220 a.

The optical modulator 335 is closed (deflects light from the path 314 or otherwise prevents incident light from remaining on the path 314) from the time t1 to the time t2. At the time t2, the optical modulator 335 begins to transition to the open state. The optical modulator 335 is open between the times t2 and t3, and, during this time range, the optical modulator 335 transmits incident light. The optical modulator 335 transitions to be closed at the time t3, and becomes closed again at the time t4. As discussed above, the transition times (the time between the time t2 and t3 and the time between t3 and t4) can be reduced by reducing the beam diameter of the light that is gated by the optical modulator 335.

Referring also to FIG. 5B, the times t2 and t3 are selected such that the pulse 510 a is incident on the optical modulator 335 at a time when the modulator 335 is open. Thus, the pulse 510 a passes through the optical modulator 335 to reach the initial target 220 a. The times t3 and t4 are selected so that the optical modulator 335 begins to close after transmitting the pulse 510 a and is closed when the reflection 513 a is incident on the optical modulator 335. In this way, the optical modulator 335 provides a time-gate based isolation of the pre-pulse reflection 513 a.

In some implementations, the beam diameter of the pre-pulse 510 a and the reflection 513 a can be 3 mm. In implementations in which the optical modulator 335 is an acousto-optic modulator, the time that the optical modulator takes to transition from open to close and vice versa is determined by the beam diameter of the incident light and the speed of sound in the material of the optical modulator. The material can be, for example, germanium (Ge), which has an acoustic wave speed of 5500 meters/second (m/s). In this example, the transition time (the time for the optical modulator to transition from closed to open) is 375 nanoseconds (ns). The delay between the pre-pulse 510 a and the reflection 513 a can be, for example, 400 ns. Thus, the pre-pulse 510 a is transmitted by the optical modulator 335 and the reflection 513 a is deflected off of the path 314.

In some implementations, the optical modulator 335 is closed except for the period of time at which the pulse 510 a is expected. By remaining closed at other times, the optical modulator 335 prevents the reflection 513 a from entering the light-generation module 204. Additionally, by remaining closed, the modulator 335 also prevents or reduces the impact of secondary reflections of the pulse 510 a. Elements, such as filters, pinholes, lenses, and tubes, on the path 112 are sources of glint and reflect incident light. These elements can reflect the pulse 510 b and cause secondary reflections that propagate on the path 112 and the path 314, and these secondary reflections are in addition to the reflection 513 a. By keeping the modulator 335 closed except when the pulse 510 a is incident on the modulator 335, the secondary reflections are also prevented from entering the light-generation module 204. Furthermore, the secondary reflections are removed from the path 314 and are thus prevented from propagating back onto the path 112. In this way, the secondary reflections cannot reach the initial target region 215 a, the modified target region 215 b, or the region between the regions 215 a and 215 b. If the secondary reflections are able to reach these regions, the reflections can harm the target by breaking it apart before the target reaches the modified target region 215 b. The secondary reflections can be referred to as forward pulse excited by reverse pulses (FERs). The optical isolator 306 can help mitigate self-lasing, which can limit the maximum about of optical power delivered to the target region 215 b.

Referring to FIG. 6, a block diagram of another exemplary optical isolator 606 is shown. The optical isolator 606 can be used instead of the optical isolator 106 in the system 100 (FIG. 1) or the system 200 (FIG. 2). Additionally, the optical isolator 606 can be used in any other optical system where the prevention of back reflections is desirable. The optical isolator 606 is discussed with respect to a configuration in which the optical isolator 606 is used as the optical isolator 106 in the system 200 (FIG. 2). The optical isolator 606 can be used with the polarization isolator 303 discussed above with respect to FIG. 3. In implementations that include the polarization isolator 303, the polarization isolator 303 is between the optical isolator 606 and the optical amplifier 108 (FIGS. 1 and 2) to provide additional blocking of back reflections.

The optical isolator 606 is similar to the optical isolator 306 (FIG. 3), except the optical isolator 606 includes a second optical modulator 637. The second optical modulator 637 is on the path 112, and is positioned between the dichroic optical element 331 and the dichroic optical element 336. Similar to the optical modulator 335, the second optical modulator 637 transmits incident light when in an open state and deflects or blocks incident light when in a closed state. The second light beam 210 b is emitted from the light-generation module 204 and propagates on the path 112 to the dichroic optical element 331.

As discussed above, the dichroic optical element 331 transmits the wavelength of the second light beam 210 b. Thus, the second light beam 210 b passes through the dichroic optical element 331 and is incident on the second optical modulator 637. The second optical modulator 637 is controlled to be in the open state when the second light beam 210 b is incident on the modulator 637, and the second light beam 210 b passes through the modulator 637 and the dichroic optical element 336, remaining on the path 112 and reaching the modified target region 215 b (FIG. 2). Part of the second light beam 210 b is reflected from the modified target 220 b (in addition to converting at least some of the target material to plasma that emits EUV light) and can propagate as a reflection 213 b along the path 112 in a direction other than the z direction.

The reflection 213 b is transmitted by the dichroic optical element 336 and remains on the path 112. The optical modulator 637 is closed when the reflection 213 b is incident on the modulator 637, and the reflection 213 b is deflected from the path 112 as deflected light 217 b. Thus, the second modulator 637 prevents the reflection 213 b from reaching the light-generation module 204 or reduces the amount of the reflection 213 b that reaches the light-generation module 204, reducing or eliminating self-lasing from the light-generation module 404 and allowing the second light beam 210 b to be of greater energy. In some implementations, the optical modulator 637 deflects 30-40% of the reflection 213 b. The time during which the optical modulator 637 is open can be reduced to further reduce the amount of self-lasing. For example, reducing the open time from 20 microseconds (μs) to 2 μs can reduce the self-lasing by 90%.

The second modulator 637 is closed except for the period of time at which the beam 210 b is expected. By remaining closed at other times, the second modulator 637 prevents the reflection 213 b from entering the light-generation module 204. Additionally, by remaining closed, the second modulator 637 also prevents or reduces the impact of secondary reflections from of the second beam 210 b. Elements, such as filters, pinholes, lenses, and tubes, on the path 112 are sources of glint and reflect incident light. These elements can reflect the second beam 210 b and cause secondary reflections that are in addition to the reflection 213 b (which is caused by an interaction between the second beam 210 b and the modified target 220 b). By keeping the modulator 637 closed except when the second light beam 210 b is incident on the modulator 637, the secondary reflections are also prevented from entering the light-generation module 204 and the secondary reflections are removed from the path 112.

The second optical modulator 637 can be the same as the modulator 335, or the second optical modulator 637 and the modulator 335 can be different types of modulators.

Referring to FIG. 7, a block diagram of a system 700 is shown. The system 700 includes a light-generation module 704, a control system 740, and an optical modulator 735. The light-generation module 704 can be the light-generation module 104 (FIG. 1), the light-generation module 204 (FIG. 2), or any other system that generates light beams having different wavelengths. The optical modulator 735 can be the optical modulator 335 (FIG. 3) and/or the optical modulator 637 (FIG. 6).

The control system 740 provides a trigger signal 747 to the optical modulator 735. The trigger signal 747 is sufficient to cause the optical modulator 735 to change state or to begin to change state. For example, in implementations in which the optical modulator 735 is an acousto-optic modulator, the trigger signal 747 can cause the modulator to transition to a closed state by causing a transducer to vibrate to form sound waves in the modulator. The control system 740 also can receive data from the light-generation module 704 through a signal 741, and can provide data to the light-generation module 704 through a signal 742. Further, the control system 740 also can receive data from the optical module 735 via a signal 742.

The control system 740 includes an electronic storage 743, an electronic processor 744, and an input/output (I/O) interface 745. The electronic processor 744 includes one or more processors suitable for the execution of a computer program such as a general or special purpose microprocessor, and any one or more processors of any kind of digital computer. Generally, a processor receives instructions and data from a read-only memory or a random access memory or both. The electronic processor 744 can be any type of electronic processor.

The electronic storage 743 can be volatile memory, such as RAM, or non-volatile memory. In some implementations, and the electronic storage 743 can include both non-volatile and volatile portions or components. The electronic storage 743 can data and information that is used in the operation of the optical modulator 735. For example, the electronic storage 743 can store timing information that specifies when the first and second beams 210 a, 210 b are expected to propagate through the system 200 (FIG. 2). The electronic storage 743 also can store instructions, perhaps as a computer program, that, when executed, cause the processor 744 to communicate with other components in the control system 740, the light-generation module 704, and/or the optical modulator 735. For example, the instructions can be instructions that cause the electronic processor 744 to provide a trigger signal 747 to the optical modulator 735 at certain times that are specified by the timing information stored on the electronic storage 743.

The I/O interface 745 is any kind of electronic interface that allows the control system 740 to receive and/or provide data and signals with an operator, the light-generation module 704, the optical modulator 735, and/or an automated process running on another electronic device. For example, the I/O interface 745 can include one or more of a visual display, a keyboard, or a communications interface.

Referring to FIG. 8A, an LPP EUV light source 800 is shown. The optical systems 100 and 200 can be part of an EUV light source, such as the source 800. The LPP EUV light source 800 is formed by irradiating a target mixture 814 at a target location 805 with an amplified light beam 810 that travels along a beam path toward the target mixture 814. The target location 805, which is also referred to as the irradiation site, is within an interior 807 of a vacuum chamber 830. When the amplified light beam 810 strikes the target mixture 814, a target material within the target mixture 814 is converted into a plasma state that has an element with an emission line in the EUV range. The created plasma has certain characteristics that depend on the composition of the target material within the target mixture 814. These characteristics can include the wavelength of the EUV light produced by the plasma and the type and amount of debris released from the plasma.

The light source 800 also includes a target material delivery system 825 that delivers, controls, and directs the target mixture 814 in the form of liquid droplets, a liquid stream, solid particles or clusters, solid particles contained within liquid droplets or solid particles contained within a liquid stream. The target mixture 814 includes the target material such as, for example, water, tin, lithium, xenon, or any material that, when converted to a plasma state, has an emission line in the EUV range. For example, the element tin can be used as pure tin (Sn); as a tin compound, for example, SnBr₄, SnBr₂, SnH₄; as a tin alloy, for example, tin-gallium alloys, tin-indium alloys, tin-indium-gallium alloys, or any combination of these alloys. The target mixture 814 can also include impurities such as non-target particles. Thus, in the situation in which there are no impurities, the target mixture 814 is made up of only the target material. The target mixture 814 is delivered by the target material delivery system 825 into the interior 607 of the chamber 630 and to the target location 605.

The light source 800 includes a drive laser system 815 that produces the amplified light beam 810 due to a population inversion within the gain medium or mediums of the laser system 815. The light source 800 includes a beam delivery system between the laser system 815 and the target location 805, the beam delivery system including a beam transport system 820 and a focus assembly 822. The beam transport system 820 receives the amplified light beam 810 from the laser system 815, and steers and modifies the amplified light beam 810 as needed and outputs the amplified light beam 810 to the focus assembly 822. The focus assembly 822 receives the amplified light beam 810 and focuses the beam 810 to the target location 805.

In some implementations, the laser system 815 can include one or more optical amplifiers, lasers, and/or lamps for providing one or more main pulses and, in some cases, one or more pre-pulses. Each optical amplifier includes a gain medium capable of optically amplifying the desired wavelength at a high gain, an excitation source, and internal optics. The optical amplifier may or may not have laser mirrors or other feedback devices that form a laser cavity. Thus, the laser system 815 produces an amplified light beam 810 due to the population inversion in the gain media of the laser amplifiers even if there is no laser cavity. Moreover, the laser system 815 can produce an amplified light beam 810 that is a coherent laser beam if there is a laser cavity to provide enough feedback to the laser system 815. The term “amplified light beam” encompasses one or more of: light from the laser system 815 that is merely amplified but not necessarily a coherent laser oscillation and light from the laser system 815 that is amplified and is also a coherent laser oscillation.

The optical amplifiers in the laser system 815 can include as a gain medium a filling gas that includes CO₂ and can amplify light at a wavelength of between about 9100 and about 11000 nm, and in particular, at about 10600 nm, at a gain greater than or equal to 800. Suitable amplifiers and lasers for use in the laser system 815 can include a pulsed laser device, for example, a pulsed, gas-discharge CO₂ laser device producing radiation at about 9300 nm or about 10600 nm, for example, with DC or RF excitation, operating at relatively high power, for example, 10 kW or higher and high pulse repetition rate, for example, 40 kHz or more. The optical amplifiers in the laser system 815 can also include a cooling system such as water that can be used when operating the laser system 815 at higher powers.

FIG. 8B shows a block diagram of an example drive laser system 880. The drive laser system 880 can be used as part of the drive laser system 815 in the source 800. The drive laser system 880 includes three power amplifiers 881, 882, and 883. Any or all of the power amplifiers 881, 882, and 883 can include internal optical elements (not shown).

Light 884 exits from the power amplifier 881 through an output window 885 and is reflected off a curved mirror 886. After reflection, the light 884 passes through a spatial filter 887, is reflected off of a curved mirror 888, and enters the power amplifier 882 through an input window 889. The light 884 is amplified in the power amplifier 882 and redirected out of the power amplifier 882 through an output window 890 as light 891. The light 891 is directed toward the amplifier 883 with a fold mirror 892 and enters the amplifier 883 through an input window 893. The amplifier 883 amplifies the light 891 and directs the light 891 out of the amplifier 883 through an output window 894 as an output beam 895. A fold mirror 896 directs the output beam 895 upward (out of the page) and toward the beam transport system 820 (FIG. 8A).

Referring again to FIG. 8B, the spatial filter 887 defines an aperture 897, which can be, for example, a circle having a diameter between about 2.2 mm and 3 mm. The curved mirrors 886 and 888 can be, for example, off-axis parabola mirrors with focal lengths of about 1.7 m and 2.3 m, respectively. The spatial filter 887 can be positioned such that the aperture 897 coincides with a focal point of the drive laser system 880.

Referring again to FIG. 8A, the light source 800 includes a collector mirror 835 having an aperture 840 to allow the amplified light beam 810 to pass through and reach the target location 805. The collector mirror 835 can be, for example, an ellipsoidal mirror that has a primary focus at the target location 805 and a secondary focus at an intermediate location 845 (also called an intermediate focus) where the EUV light can be output from the light source 800 and can be input to, for example, an integrated circuit lithography tool (not shown). The light source 800 can also include an open-ended, hollow conical shroud 850 (for example, a gas cone) that tapers toward the target location 805 from the collector mirror 835 to reduce the amount of plasma-generated debris that enters the focus assembly 822 and/or the beam transport system 820 while allowing the amplified light beam 810 to reach the target location 805. For this purpose, a gas flow can be provided in the shroud that is directed toward the target location 805.

The light source 800 can also include a master controller 855 that is connected to a droplet position detection feedback system 856, a laser control system 857, and a beam control system 858. The light source 800 can include one or more target or droplet imagers 860 that provide an output indicative of the position of a droplet, for example, relative to the target location 805 and provide this output to the droplet position detection feedback system 856, which can, for example, compute a droplet position and trajectory from which a droplet position error can be computed either on a droplet by droplet basis or on average. The droplet position detection feedback system 856 thus provides the droplet position error as an input to the master controller 855. The master controller 855 can therefore provide a laser position, direction, and timing correction signal, for example, to the laser control system 857 that can be used, for example, to control the laser timing circuit and/or to the beam control system 858 to control an amplified light beam position and shaping of the beam transport system 820 to change the location and/or focal power of the beam focal spot within the chamber 830.

The target material delivery system 825 includes a target material delivery control system 826 that is operable, in response to a signal from the master controller 855, for example, to modify the release point of the droplets as released by a target material supply apparatus 827 to correct for errors in the droplets arriving at the desired target location 805.

Additionally, the light source 800 can include light source detectors 865 and 870 that measures one or more EUV light parameters, including but not limited to, pulse energy, energy distribution as a function of wavelength, energy within a particular band of wavelengths, energy outside of a particular band of wavelengths, and angular distribution of EUV intensity and/or average power. The light source detector 865 generates a feedback signal for use by the master controller 855. The feedback signal can be, for example, indicative of the errors in parameters such as the timing and focus of the laser pulses to properly intercept the droplets in the right place and time for effective and efficient EUV light production.

The light source 800 can also include a guide laser 875 that can be used to align various sections of the light source 800 or to assist in steering the amplified light beam 810 to the target location 805. In connection with the guide laser 875, the light source 800 includes a metrology system 824 that is placed within the focus assembly 822 to sample a portion of light from the guide laser 875 and the amplified light beam 810. In other implementations, the metrology system 824 is placed within the beam transport system 820. The metrology system 824 can include an optical element that samples or re-directs a subset of the light, such optical element being made out of any material that can withstand the powers of the guide laser beam and the amplified light beam 810. A beam analysis system is formed from the metrology system 824 and the master controller 855 since the master controller 855 analyzes the sampled light from the guide laser 875 and uses this information to adjust components within the focus assembly 822 through the beam control system 858.

Thus, in summary, the light source 800 produces an amplified light beam 810 that is directed along the beam path to irradiate the target mixture 814 at the target location 805 to convert the target material within the mixture 814 into plasma that emits light in the EUV range. The amplified light beam 810 operates at a particular wavelength (that is also referred to as a drive laser wavelength) that is determined based on the design and properties of the laser system 815. Additionally, the amplified light beam 810 can be a laser beam when the target material provides enough feedback back into the laser system 815 to produce coherent laser light or if the drive laser system 815 includes suitable optical feedback to form a laser cavity.

Referring to FIG. 9, a plot 900 of example test data for an optical isolator such as the optical isolator 306 (FIG. 3) is shown. The plot 900 shows the measured power of a reverse-going pre-pulse beam as a function of time with the optical isolator in an ON state and in an OFF state. The reverse-going pre-pulse beam can be a beam such as the reflection 213 a (FIG. 2), which arises from the interaction between the first beam 210 a (FIG. 2) and the initial target 220 a (FIG. 2) as discussed above. In the ON state, the optical isolator blocks or reduces the effects of the reflection 213 a by deflecting all or part of the reflection 213 a from the beam path 314 so that the reflection 213 a that reaches the light-generation module 204 is reduced or eliminated. In the ON state, the optical isolator can operate, for example, as discussed with respect to FIGS. 5A and 5B. In the OFF state, the optical isolator is not active and the system operates as if the optical isolator is not present.

In the example of FIG. 9, the optical isolator is in the OFF state between the times 905 and 910, and otherwise is in the ON state. When the optical isolator is in the ON state, the power of the reflection 213 a that reaches the light-generation module 204 is very low, and is close to zero Watts (W). For example, the power of the reflection 213 a that reaches the light-generation module 204 can be about or below 0.1 W. As discussed above, it is desirable to reduce the power of the reflection 213 a that reaches the light-generation module 204. In contrast, when the optical isolator is in the OFF state, the power of the reflection 213 a that reaches the light-generation module 204 is greater than 0 and can be between about 4.2 W and 18.2 W. Furthermore, when the optical isolator is in the OFF state, the power of the reflection 213 a that reaches the light-generation module 204 varies quite a bit, which can lead to instabilities in the system. Thus, in addition to reducing the amount of power in the reflection 213 a, the optical isolator also reduces the variation of the power of the reflection, resulting in a more stable system.

Referring to FIGS. 10A and 10B, additional example test data are shown. FIG. 10A shows the energy of the produced EUV light as a function of pulse number when the optical isolator (such as the optical isolator 306) is not present in the system, and FIG. 10B shows the energy of the produced EUV light as a function of pulse number when the optical isolator is present in the system. When the optical isolator is not present, the average energy of the EUV light is 3.4 milliJoules (mJ). When the optical isolator is present, the average EUV energy increases to 4.1 mJ.

Referring also to FIGS. 11A and 11B, the produced EUV light is also more stable when the optical isolator is present in the system. FIG. 11A shows a distribution of particular values of the energy of the produced EUV light when the optical isolator is not present, and FIG. 11B shows the distribution of particular values of the energy of the produced EUV light when the optical isolator is present. The distribution of energy values of FIG. 11B (when the optical isolator is used) shows that higher energy values occur more often and that all of the energy values are contained in a smaller range as compared to a system that does not employ the optical isolator. Thus, using an optical isolator (such as the optical isolator 306) results in EUV light of higher energy and also results in EUV light that is more stable (varies less).

Referring to FIGS. 12A-12C and 13A-13C, additional example test data are shown. FIGS. 12A-12C show a target 1200 at three times in a system that lacks an optical isolator such as the optical isolator 306, and FIGS. 13A-13C show a target 1300 at three times in a system that includes an optical isolator such as the optical isolator 306. The targets 1200 and 1300 include the target material that emits EUV light when in a plasma state. The targets 1200 and 1300 are shown at times that coincide with the targets 1200 and 1300 being in a location that receives a pre-pulse (such as the initial target region 215 a of FIG. 2) and a location that receives a main pulse (such as the modified target region 215 b of FIG. 2).

As discussed above with respect to FIGS. 5A and 5B, the optical isolator can reduce or eliminate secondary reflections from objects such as pin holes, lenses, tubes, and optical elements. When present, the secondary reflections can reach the target as it moves from the initial target region 215 a to the modified target region 215 b. FIGS. 12A-12C show an example of the secondary reflections interacting with the target 1200 over time. As shown in FIGS. 12B and 12C as compared to 12A, the target 1200 spreads out spatially as time passes and breaks apart. FIGS. 13A-13C show an example of a system that uses the optical isolator (such as the optical isolator 306) to reduce or eliminate the secondary reflections. As compared to the target 1200 (FIGS. 12A-12C), the target 1300 (FIGS. 13A-13C) has a cleaner spatial profile, which can lead to increased absorption of an incident light beam and more target material available for interaction with the second beam 210 b (and thus more EUV light produced). Additionally, because the target 1300 is used with an optical source that includes the optical isolator, the target 1300 can have a flat orientation relative to the direction of propagation of the incident light beam while still reducing or eliminating the effects of back reflections and secondary reflections on the optical source.

Other implementations are within the scope of the claims.

In implementations in which the optical subsystems 204 a, 204 b (FIG. 2) are different types of optical subsystems, the optical subsystem 204 a can be a rare-earth-doped solid state laser (such as a Nd:YAG or an erbium-doped fiber (Er:glass)), and the wavelength of the first light beam 210 a can be 1.06 μm. The optical subsystem 204 b can be a CO₂ laser, and the wavelength of the light beam 210 b can be, for example, 10.26 μm. In these implementations, the first and second beams 210 a, 210 b can be amplified in separate optical amplifiers and can follow separate paths through the system 200. Also, two separate optical isolators can be used, one for the first light beam 210 a and its corresponding reflections, and another for the light beam 210 b and its corresponding reflections.

The pre-amplifier 207 (FIG. 2) can have multiple stages. In other words, the pre-amplifier 207 can include more than one amplifier in series and placed on the path 112.

The light beams 110, 210 a, and 210 b can be pulsed light beams. The power of a pulse of the first light beam 210 a (or the pulse 510 a) can be, for example, 20-40 Watts (W). The power of a pulse of the second light beam 210 b can be, for example, 300-500 W.

The first beam of light 210 a can be any type of radiation that can act on the initial target 220 a to form the modified target 220 b. For example, the first beam of light 210 can be a pulsed optical beam generated by a laser. The first beam of light 210 can have a wavelength of about 1-10.6 μm. The duration of a pulse of the first beam of light 210 a can be, for example, 20-70 nanoseconds (ns), less than 1 ns, 300 picoseconds (ps), between 100-300 ps, between 10-50 ps, or between 10-100 ps. The energy of a pulse of the first beam of light 210 a can be, for example, 15-60 milliJoules (mJ). When the pulse of the first beam of light 210 a has a duration of 1 ns or less, the energy of the pulse can be 2 mJ. The time between a pulse of the first light beam 210 a and a pulse of the second light beam 210 b can be, for example, 1-3 microseconds (μs).

The initial target 220 a and the target 115 can have any the characteristics of the target mixture 814. For example, the initial target 220 a and the target 115 can include tin.

The optical systems 100 and 200 can include the polarization isolator 303. In these implementations of the optical system 100, the polarization isolator 303 is between the optical isolator 106 and the optical amplifier 108. 

What is claimed is:
 1. An apparatus for reducing an amount of light reflected toward an optical source of an extreme ultraviolet (EUV) light source, the apparatus comprising: a plurality of dichroic optical elements, each of the dichroic optical elements being configured to reflect light having a wavelength in a first band of wavelengths and to transmit light having a wavelength in a second band of wavelengths, and the plurality of dichroic optical elements comprising at least a first dichroic optical element and a second dichroic optical element; a plurality of other optical elements on a first beam path between the first dichroic optical element and the second dichroic optical element, the other optical elements positioned to interact with light reflected from the first dichroic optical element and/or the second dichroic optical element; and an optical modulator positioned on the first beam path between the first dichroic optical element and the second dichroic optical element, wherein the first band of wavelengths comprises a wavelength of a first light beam, the first light beam having an energy sufficient to modify a geometric distribution of target material in an initial target to form a modified target, the target material emitting EUV light when in a plasma state, and the second band of wavelengths comprises a wavelength of a second light beam, the second light beam having an energy sufficient to convert at least some of the target material in the modified target to the plasma that emits EUV light.
 2. The apparatus of claim 1, wherein the optical modulator is configured to transmit light that propagates in a first direction on the first beam path and to deflect light that propagates in a second direction on the first beam path, and the second direction is different from the first direction.
 3. The apparatus of claim 2, wherein the optical modulator is configured to deflect light that propagates in the second direction away from the first beam path.
 4. The apparatus of claim 1, wherein the optical modulator comprises an acousto-optic modulator.
 5. The apparatus of claim 4, wherein the apparatus further comprises a control system configured to provide a trigger signal to the acousto-optic modulator, and the acousto-optic modulator deflects light in response to receiving the trigger signal and otherwise transmits light onto the first beam path.
 6. The apparatus of claim 5, wherein the acousto-optic modulator deflects light away from the first beam path in response to receiving the trigger signal. The apparatus of claim 1, further comprising a second optical modulator.
 8. The apparatus of claim 7, wherein the second optical modulator is between the first dichroic optical element and the second dichroic optical element, the second optical modulator is positioned to receive light transmitted by the first dichroic optical element and transmitted by the second dichroic optical element, and the second optical modulator is on a second beam path that is different from the first beam path.
 9. The apparatus of claim 1, wherein the first beam of light comprises a first pulsed beam of light, and the second beam of light comprises a second pulsed beam of light.
 10. The apparatus of claim 1, wherein the plurality of other optical elements comprises a plurality of reflective optical elements, and the first dichroic optical element, the second dichroic optical element, and one or more of the reflective optical elements define the first beam path.
 11. The apparatus of claim 1, wherein the optical modulator comprises an acousto-optic modulator, the acousto-optic modulator being associated with a first state and a second state, the acousto-optic modulator deflecting incident light from an initial beam path in the first state and transmitting incident light onto the initial beam path in the second state, and the optical isolator further comprises a control system configured to provide a trigger signal to the acousto-optic modulator, the acousto-optic modulator being configured to change from one of the first state and the second state to the other of the first state and the second state in response to receiving the trigger signal.
 12. A method of reducing an amount of light reflected toward an optical source of an extreme ultraviolet (EUV) light source, the method comprising: receiving a first light beam at a first dichroic optical element, wherein the first light beam is produced by an optical source and has a wavelength in a first band of wavelengths, and an interaction between the first light beam and the first dichroic optical element directs the first light beam onto a first beam path and toward an optical modulator; controlling the optical modulator to thereby direct the first light beam toward a second dichroic optical element and toward a target location; receiving a reflection of the first light beam from the target location at the second dichroic optical element, an interaction between the reflection of the first light beam and the second dichroic optical element directing the reflected first light beam to the optical modulator; and controlling the optical modulator to thereby direct the reflection of the first light beam away from the optical source, wherein the first and second dichroic optical elements are configured to reflect light having a wavelength in a first band of wavelengths and to transmit light having a wavelength in a second band of wavelengths.
 13. The method of claim 12, wherein controlling the optical modulator to thereby direct the reflection of the first light beam away from the optical source comprises providing a trigger signal to the optical modulator, the trigger signal being sufficient to cause the optical modulator to change state.
 14. The method of claim 13, wherein controlling the optical modulator to thereby direct the reflection of the first light beam away from the optical source comprises providing the trigger signal to the optical modulator after the first light beam passes through the optical modulator and before the reflection of the first light beam reaches the optical modulator.
 15. The method of claim 14, wherein the trigger signal causes the optical modulator to be in a state in which the optical modulator deflects incident light.
 16. The method of claim 12, wherein controlling the optical modulator to thereby direct the first light beam toward a second dichroic optical element and toward a target location comprises controlling the optical modulator such that the optical modulator transmits the first light beam.
 17. The method of claim 12, wherein the reflection of the first light beam is produced by an interaction between the first light beam and a reflective target at the target location.
 18. The method of claim 12, wherein the first light beam has an energy sufficient to modify a geometric distribution of target material in an initial target to form a modified target, and the method further comprises: receiving a second light beam at the first dichroic optical element, the second light beam having a wavelength in the second band of wavelengths and an energy sufficient to convert at least some of the target material in the modified target material to plasma that emits EUV light, and wherein an interaction between the second light beam and the first dichroic optical element directs the second light beam toward the second dichroic optical element but not toward the optical modulator.
 19. The method of claim 18, wherein directing the second light beam toward the second dichroic element further comprises directing the second light beam toward a second optical modulator, the second optical modulator being on a second beam path between the first dichroic optical element and the second dichroic optical element, the second beam path being spatially distinct from the first beam path.
 20. The method of claim 19, further comprising: receiving a reflection of the second light beam at the second dichroic optical element and at the second optical modulator; and controlling the second optical modulator to thereby direct the reflection of the second light beam away from an optical source that produces the second light beam. 